The performance of a diesel engine after-treatment system in converting NOx to other products by selective catalytic reduction (SCR) relies on the presence of ammonia in the exhaust stream. Dosing engine exhaust by injection of aqueous urea, an ammonia-based reductant, into the exhaust stream at a location upstream of an SCR catalyst is one way to introduce ammonia into the exhaust system.
It is important for injected urea solution to completely evaporate because incomplete evaporation can lead to undesired consequences such as the formation of solid deposits in the exhaust system that can adversely affect the life and performance of the after-treatment system.
A known design practice places a urea injector at a location in the engine exhaust system where it can spray urea solution into the exhaust stream ahead of the SCR catalyst with the expectation that the injected liquid will completely evaporate by the time it reaches the catalyst. Because the urea injector is placed at the exhaust system, it absorbs heat from the passing exhaust gases. If the injector is downstream of a diesel particulate filter (DPF), it is at times (e.g., during active regeneration of the DPF, either controlled or uncontrolled) exposed to significantly elevated exhaust gas temperatures. In order to limit injector temperature rise, liquid coolant from the engine cooling system is constantly flowed through internal coolant passages in the injector.
A urea injection system also typically comprises a tank for holding a supply of aqueous urea and a supply pump for pumping solution from the tank to the injector. In addition to thermal management of the urea injector, thermal management of the pump and the tank is important because in a motor vehicle such as a truck the latter two components are typically mounted on the motor vehicle chassis where the urea solution is exposed to ambient temperature. In cold ambient temperatures near and below about 12° F., the liquid in the tank, pump, and associated conduits can freeze while in hot ambient temperatures, the solution can become unstable, significantly reducing its effectiveness when injected into the after-treatment system.